1. Field of the Invention
The present invention relates generally to specimen inspection.
More particularly, the present invention relates to e-beam inspection systems.
2. Description of the Background Art
Semiconductor manufacturing processes include deposition and etching of various material layers on a semiconductor wafer. During the processing, various microscopic features (trenches, islands, and so on) are created on the wafer. Often times, the cross-sectional profile of a microscopic feature may be of interest to the manufacturer. In particular, the angle of undercut of a sidewall of the feature may be of interest.
Scanning electron microscopy (SEM) may be used to inspect a wafer, and the rock angle of the incident beam may be varied in an attempt to view an undercut. Unfortunately, SEM images tend to have significant resolution degradation when taken at large rock angles. This blurring of the images makes impractical the determination of large undercut angles by viewing SEM images at large rock angles.
Another conventional technique for determining undercut angles is by way of focus ion beam (FIB) sections. FIB systems impinge a focused beam on ions (for example, gallium ions) onto a specimen. The focused ion beam may act to precision mill the specimen at high beam currents or to image the specimen at low beam currents (in which case less material is sputtered). Hence, an FIB system may be used in preparing a cross-section specimen for transmission electron microscope (TEM) imaging. Recent FIB systems may be utilized for in-situ cross-section preparation and high-resolution imaging. However, FIB techniques are disadvantageously destructive due to the sputtering or milling of material from the sample.
FIG. 1A is a conventional image of a cross section of a feature 170 that is slightly undercut on both left and right sides. The image of the cross section was obtained by the conventional focused ion beam (FIB) technique. As mentioned above, the FIB technique is disadvantageous in that it requires destruction of the specimen. This is because the FIB technique thins the sample by ion milling.
FIG. 1B shows a conventional analysis of the cross-sectional FIB image of the feature 170 to determine the undercut angles. The analysis gives an outline of the feature 170. Using the outline of the feature 170, the undercut angle may be determined by comparing the actual left 172-L and right 172-R sidewalls to vertical reference lines 174-L and 174-R, respectively. (The slight asymmetry seen in the reference lines is thought to be due to the milling of the sample.) Analysis of this FIB image indicates a left undercut of about five (5) degrees and a right undercut of about two (2) degrees.